Gold Nanoparticle-Based pH Sensor in Highly Alkaline Region at pH > 11: Surface-Enhanced Raman Scattering Study

Abstract

A surface plasmon resonance spectroscopy study showed that citrate-reduced gold nanoparticles (∼15 nm diameter, ∼9 × 10⁻⁹ M concentration, ∼2 × 10⁻² M ionic strength) were found to be utilized as a colorimetric sensor by exhibiting a distinct color change at a highly alkaline pH > 11.5. Surface-enhanced Raman scattering (SERS) of 4-ethynylpyridine (4-EP) on gold nanoparticle surfaces indicated that the multiple peaks in the v(C≡C) stretching bands should vary significantly in the highly alkaline region from pH 12 to 14. As the pH value increased, the v(C≡C) stretching band intensity at ∼2080 cm⁻¹ became stronger than that at ∼2010 cm⁻¹. The pK_1/2 value was determined to be around 13 by the SERS titration of taking intensity ratios of I₂₀₈₀ with respect to I₂₀₁₀. Using SERS enhancements and conspicuous spectral changes, self-assembled monolayers (SAMs) of 4-EP on Au nanoparticles holds potential as a pH sensor for sensitive detection of the hydroxide OH⁻ concentration at around pH 13 in an aqueous solution. The pH calibration from SERS titration of 4-EP is expected to have advantages in terms of higher alkaline detection limit and more precise measurements, if compared with the indigo carmine, the pK_1/2 value of which is 12.2.